a) Field of the Invention
This invention relates to a system and method for controlling shifting of a vehicle automatic transmission so that an engine brake can be applied to a vehicle especially when the vehicle is running on a meandering downhill road.
b) Description of the Related Art
A conventional vehicle automatic transmission suitable for use on an automotive vehicle is designed to perform a change-over in speed range such as an upshift or a downshift on the basis of a preset shift pattern upon receipt of information on an engine load such a throttle opening, information on a vehicle speed and information on a current speed range.
Under normal shift control, such a conventional vehicle automatic transmission involves no particular serious problem in shifting the speed range on a flat road as in street running, whereby the shifting is smooth and gives no sense of incongruity. When running, for example, in a mountainous region, however, there are straight uphill roads and also frequently bent uphill roads as well as steep downhill slopes and meandering downhill slopes requiring a strong engine brake, and gentle long downhill slopes. Appropriate smooth shifting is not always assured.
While running in such a mountainous region, it is rather difficult to choose a speed range optimal to the state of running of the vehicle, the driving behavior intended by the driver, road conditions and the like. There is accordingly a demand for permitting good control of motion of a vehicle by a simple running operation even during running in a mountainous region so that better drive feeling and run feeling can be obtained. On zig-zag downhill roads often found in mountainous regions, in particular, driving would be facilitated further if it would become possible to select such a speed range that applies an adequate engine brake.
For such a demand, it is known, for example, from Japanese Patent Application Laid-Open (Kokai) No. SHO 62-246546 or HEI 2-3738 to perform the so-called "fuzzy control" so that an optimal speed range corresponding to the above-described state of running of the vehicle can be selected.
These conventional shift control methods are designed to set optimal speed ranges by inferring all gear positions for running in both urban and mountainous regions in accordance with fuzzy inference. These conventional shift control methods making use of "fuzzy control" are however accompanied by the problem that they require many rules and hence a membership function of an intricate profile. This has led to the problems that a large-capacity computer is needed to put such a method into practical application and tuning of the control is difficult, thereby making it difficult to apply the control method to other types of automotive vehicles.
Further, additional incorporation of shift control by "fuzzy control" may result in the execution of shifting by such a small change in the running or driving state, e.g., running-over of a small bump or slight depression of an accelerator pedal that no shifting would take place according to the conventional art. This has led to the problem that the above shifting may give a sense of incongruity to drivers who are accustomed to running on flat roads such as street running under control by a conventional automatic transmission.
It is desired to surely determine each meandering downhill road and further to adequately apprehend the driver's demand for an engine brake on the meandering downhill road. Shift control by conventional "fuzzy control" has however not attained it to any satisfactory extent.
On such meandering downhill roads, driving would be made still easier if it would become feasible to achieve selection of such a speed range as permitting application of an adequate engine brake.
In general, it may be considered to determine the driver's demand for an engine brake on the basis of information on a braking operation by the driver. Proposals which relate to shift control and use information on a braking operation by a driver include, for example, those disclosed in Japanese Patent Applications Laid-Open (Kokai) Nos. HEI 2-3738 and HEI 1-269749. The former proposal uses information on a braking operation only for the prevention of a false judgment upon performing processing in a computer and is not relevant directly to shift control. The latter proposal determines the frequency of braking operations from information on the braking operations and then uses the frequency as a basis for the determination of whether the vehicle is running in a residential area or not. The frequency of braking operations, however, cannot be determined instantaneously from the driver's action and further, cannot be used directly for the determination of a meandering downhill road.